fix: word the cases-folder refusal for Clone Repo too (release review)

Clone Repo's picker now offers New folder, and with the field empty Browse
opens in the cases folder, so the first press there lands on this refusal.
It pointed at Add Case > Create New, which cannot clone. The sentence has to
read right from every picker that offers the button, so it names both tabs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/api-reference.md

@@ -905,10 +905,10 @@ html and svg are the ones deliberately kept out.
 `POST /api/filesystem/mkdir` with `{ path, name, sessionId?, showHidden? }` makes one
 folder directly inside `path`, which must be a folder the path picker may browse, and
 answers `{ path }`. `name` is one path segment (letters, numbers, spaces and `_ - . ~`, not
-starting with a dot or `~`). `400` for a cases folder (use Add Case → Create New with no
-custom folder) or a `path` that is not a directory, `403` for a blocked tree or a `path`
-outside the browse roots, `404` for a `path` that does not exist, `409` when the folder
-already exists, and `422` when the mount under `path` does not answer.
+starting with a dot or `~`). `400` for a cases folder (use Add Case → Create New or Clone
+Repo with no custom folder) or a `path` that is not a directory, `403` for a blocked tree
+or a `path` outside the browse roots, `404` for a `path` that does not exist, `409` when
+the folder already exists, and `422` when the mount under `path` does not answer.
 
 ## Rendered HTML files